1. Field of the Invention
The present invention relates to an exhaust device of a two-cycle engine for a motorcycle.
2. Description of Background Information
It is well known that, in a two-cycle engine the positive pressure of a pulsating wave of the exhaust gas, reflected from an opening of the exhaust pipe is applied to the exhaust port during an end period of the exhaust cycle especially when the engine is operating at a high speed, thereby pushing back the combustion mixture which is coming out at the exhaust pipe, into the combustion chamber, to raise the engine output power.
However, if the size and the form of the exhaust pipe are designed for the efficiency at a high speed operation of the engine, the effect of raising the engine output power by the above process is limited only to the high speed range of the engine operation, and even a power down is resulted in the low speed operating range because a negative pressure of the pulsating wave is applied to the exhaust port and the combustion mixture is sucked out from the combustion chamber.
In order to eliminate this drawback, it has been proposed recently to provide an exhaust device which includes a subchamber branching off from the exhaust pipe or from a portion continuing from the exhaust port of the engine, and a control valve disposed at an entrance portion of the subchamber so as to open and close the path to the subchamber.
In this kind of exhaust device, the control valve is opened when the engine speed is low, so as to change the negative pressure of the pulsating wave at the exhaust port during the end of the period of the exhaust cycle into the positive pressure by means of the effect of the subchamber, to raise the engine output power. When the engine speed is high, the control valve is closed to eliminate the effect of the subchamber to prevent power reduction of the engine.
In mounting this type of exhaust device on a motorcycle, a care is needed for the layout because numbers of devices and parts are already arranged closely with each other on a frame having a limited dimension, so as not to produce a vacant space. Therefore, it is generally conceivable to dispose the subchamber in the side of the fuel tank or directly under the fuel tank. However, in the case of the former arrangement, there are some drawbacks such that the appearance of the motorcycle is spoiled or the driving position of a rider of the motorcycle is limited by the presence of the subchamber because of the necessity of keeping the knee of the rider away from the hot subchamber. In the case of the latter arrangement, there is also a drawback that the capacity of the fuel tank must be reduced because of a space required for the subchamber.
Furthermore, when, as described in Japanese Utility Model Provisional Publication No. 56-81123, the subchamber and a communicating tube portion formed between the exhaust port and the subchamber are made continuously with the body of the cylinder, these portions extending from the cylinder become long and that causes a difficulty in mounting the engine in the frame. Moreover, during the machining of the body of the cylinder, the workability of the machine has been reduced due to the presence of the elongated exhaust port portion.